Advanced Research WRF (ARW) Technical Note - MMM - University ...

5.2.3 Generating Lateral Boundary Data
This section deals with generating the separate lateral boundary condition file used exclusively
for the real-data cases. For information on which lateral boundary options are available for both
the idealized and real-data cases, see Chapter (6).
The specified lateral boundary condition for the coarse grid for real-data cases is supplied
by an external file that is generated by program real. This file contains records for the fields u,
v, θ, qv, φ ′ , and µ ′ d
that are used by the ARW to constrain the lateral boundaries (other fields
are in the boundary file, but are not used). The lateral boundary file holds one less time period
than was processed by the SI. Each of these variables has both a valid value at the initial time
of the lateral boundary time and a tendency term to get to the next boundary time period. For
example, assuming a 3-hourly availability of data from the SI, the first time period of the lateral
boundary file for u would contain data for both coupled u (map scale factor and µd interpolated
to the variable’s staggering) at the 0 h time
and a tendency value defined as
U0h = µd x u
m x
Ut = U3h − U0h
,
3h
which would take a grid point from the initial value to the value at the next large-scale time
during 3 simulation hours. The horizontal momentum fields are coupled both with µd and the
inverse map factor. The other 3-dimensional fields (θ, qv, and φ ′ ) are coupled only with µd. The
µ ′ d lateral boundary field is not coupled.
Each lateral boundary field is defined along the four sides of the rectangular grid (loosely
referred to as the north, south, east, and west sides). The boundary values and tendencies for
vertical velocity and the non-vapor moisture species are included in the external lateral boundary
file, but act as place-holders for the nested boundary data for the fine grids. The width of the
lateral boundary along each of the four sides is user selectable at run-time.
5.2.4 Masking of Surface Fields
Some of the meteorological and static fields are “masked”. A masked field is one in which the
values are typically defined only over water (e.g., sea surface temperature) or defined only over
land (e.g., soil temperature). The need to match all of the masked fields consistently to each
other requires additional steps for the real-data cases due to the masked data’s presumed use in
various physics packages in the soil, at the surface, and in the boundary layer. If the land/water
mask for a location is flagged as a water point, then the vegetation and soil categories must also
recognize the location as the special water flag for each of their respective categorical indices.
The values for the soil temperature and soil moisture come from the SI on the native levels
originally defined for those variables in the large-scale model. The SI does no vertical interpolation
for the soil data. While it is typical to try to match the ARW soil scheme with the
incoming data, that is not a requirement. Pre-processor real will vertically interpolate (linear in
depth below the ground) from the incoming levels to the requested soil layers to be used within
the model.
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0h
,